The main purpose of designing cement slurry for extreme and deep environment (HPHT wells) is to develop high performance cement system in well bore to achieve zonal isolation. The primary objective of cement slurry is to improve rheological properties and displacement efficiency of cement system. Oil well slurries depend on its homogeneity of additive concentrations, quality and quantity to contribute the placement and success of a well drilling cementing operation. This research study is focused on the laboratory study of the High Performance Cement System (HPCS). This investigation of cement slurry was prepared with Silica Fume (SF) and excess amount of water to decrease the slurry density in order to observe the rheological properties above 120C at different concentration of SF. Results indicates that the designed cement rheological properties are directly influenced by the shear rate and shear stress on the pump-ability of the cement with the increase of the SF concentration for the rheological improvement.